In Vivo Measurement of the Human Crystalline Lens Equivalent Refractive Index using Whole-eye OCT

Abstract

Purpose : Knowledge of the equivalent index of the lens and its age-dependence can provide insight into age-related changes in the lens refractive index (RI) distribution. Calculating the equivalent RI generally requires the use of multiple biometry devices which increases measurement uncertainty. The purpose of this study is to evaluate the feasibility of determining the equivalent index in vivo using a single OCT system that generates images of the whole eye.

Methods : Following an IRB-approved protocol, the left eye of 8 subjects (40.1 ± 14.8 years old, spherical equivalent refraction ranging from -3 to 0 D) was imaged in the unaccommodated state using a custom-built extended depth OCT imaging system with an axial resolution of 8 µm. The system generates cross-sectional images consisting of 400 A-lines, ranging in depth from the cornea to the retina and at a rate of 8.7 frames per second (Ruggeri et al, Biomed Opt Express 3:1506-1520; 2012). Boundaries of the cornea, lens, and retina were automatically segmented from the OCT images. Central corneal thickness, anterior chamber depth, lens thickness, vitreous chamber depth, and anterior and posterior corneal and lens curvature were calculated from the segmentation data (Fig 1). Using a method based on Scheimpflug imaging (Dubbelman et al, Vision Research 41:1867-1877; 2001), an optimal equivalent RI was determined for each subject that minimized the error between the manifest spherical equivalent refraction and the refraction calculated using an eye model based on the data from biometry.

Results : The measured equivalent RI values ranged from 1.395 to 1.440 with a mean value of 1.415 ± 0.016, across subjects. The equivalent RI decreased with age (RI = -0.0008*age + 1.4467, p = 0.048) (Fig 2). The values attained and relationship between the equivalent RI and age are in agreement with the findings previously published by Dubbelman.

Conclusions : The study demonstrates the feasibility of quantifying the crystalline lens equivalent RI from in vivo OCT images.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Fig 1: Whole eye OCT image. Ocular distances of the subjects and ocular surface curvatures for the anterior and posterior cornea, and anterior and posterior lens were measured from OCT images.

 

Fig 2: Equivalent refractive index as a function of age. The equivalent RI decreased with age (RI = -0.0008*age + 1.4467, p = 0.048).